the global ism - ucsbweb.physics.ucsb.edu/~phys233/w2016/clm_lecture15_w2016... · 2016. 3. 9. ·...

Winter 2016 Diffuse Universe -- C. L. Martin

The Global ISM References: Draine ch 30 (2 phase medium) & ch 29 (HI properties) Draine 39.4 (3 phase medium) Also

Dopita ch 14 Tielens ch 8 Spitzer


Gas Phases •  Most of the Milky Way ISM is neutral gas.

– What temperature do we expect HI to have? – We will learn that more than one equilibrium

temperature is possible. –  This model is known as the 2-phase ISM.

•  Observed Components of the ISM •  ISM is a dynamic place.

–  Shocks heat, churn, and compress gas. –  Dense clouds give birth to stars. –  Stars heat the gas and generate shocks. –  The model that includes this hot gas is known

as the 3-phase model of the ISM.


Components of the ISM

•  Note P/k ~ 3000 K cm-3 for CNM, WNM, WIM, and HIM •  This pressure is simply a boundary condition for self-gravitating

clouds; and most self-gravitating clouds turn out to be molecular clouds. Some diffuse molecular clouds may not be self-gravitating.

•  HII regions are over-pressured. They are small and expanding.


Milky Way


Phases of the Interstellar Medium •  Gas phases characterized by different

temperatures, densities, and ionization fractions •  A stability analysis explains the coexistence of

multiple phases •  In general, a stable phase reflects the onset of a

new cooling mechanism or the decline of a heating source –  Cold HI clouds and the warm intercloud medium result

from the increased importance of [CII] cooling at higher densities and Lya and [OI] 6300 cooling at higher T.

–  The hot phase reflects the recent input of supernova energy

–  Cold molecular clouds result from the increased cooling due to rotational transitions in molecules


Heating and Cooling of the Neutral, Atomic Medium

•  At low temperature in diffuse clouds, the dominant cooling process is _____ .

•  Possible heating sources –  Far-UV photons (EUV photons absorbed in HII

regions) –  Cosmic rays –  Turbulence (supernova, galactic differential

rotation, protostellar outflows)

Cooling Rate in Neutral HI Gas


For T < 104 K, the cooling is dominated by ______ . HI Lya becomes an important coolant at T > _____ .


The Two-Phase Medium •  Field 1965 ApJ 142, 531 •  Field, Goldsmith, & Habing 1969 ApJ 155, L149 •  Cosmic-ray ionization was assumed to be the

dominant heating agent. •  To achieve a temperature of 80 K, typical of CNM

clouds, the CR ionization rate has to be 15 times larger than that favored by more recent measurements. –  Dishoeck & Black 1986

•  Nonetheless, because the heating due to CRs has a simple form independent of density and temperature, we use this old model for illustration

•  [BB]


Modern Two-Phase Medium •  If cosmic ray heating comes up short, then

what process dominate the heating of the neutral medium?

•  One might guess ionization of C. But it turns out there’s another process that’s about 1000 times more efficient.

•  The heating of diffuse, atomic clouds is dominated by the photo-electric effect on large molecules and small dust grains


Photoelectric Heating •  FUV photons absorbed by grain; electrons with a

few eV diffuse through the grain and lose energy through collisions; if the overcome the work function of the grain, then they are injected into the gas


Photoelectric Heating •  Small grains, a.ka. Large molecules, are the

most effective source of photo-electric heating –  Dominate the FUV absorption –  Photo-electons escape most easily from planar

PAH’s •  Heating is also more effective when the grains

have a low charge. [See Tielens fig 3.4] •  The grain charge is a balance between photo-

ionization and e- recombination. •  For small charge, the heating nΓ grows with G0n •  For large charge, the heating nΓ grows with nne

through the recombination rate

Steady-state Temperature



Three-phase Model •  McKee & Ostriker 1977 •  Supernova pump energy into the

the ISM •  The very hot bubbles are not

really a stable phase; recall the T1/2 dependence of free-free cooling rate

•  The long cooling time, however, means that a significant volume of the ISM may be filled with Hot Ionized (Intercloud) Medium. –  For T > 106 K; low density gas

takes over 1 Myr to cool •  Estimated porosity Q [BB]


Three-phase Model •  Predicts that supernova

remnants overlap, thereby forming a volume filling HIM, at a pressure P/k ~ 6000 cm-3 K. –  Remarkable agreement with the

observed thermal pressure. •  Fails to predict a substantial

warm HI component (only ~ 4.3% of the HI mass is WNM or WIM). –  21 cm observations indicate that

more the 60% of the HI within 500 pc of the Sun is actually in the warm phase.


Feedback from Massive Stars •  The momentum flux left over at the end of a SNe’s

radiative expansion phase creates a turbulent pressure. [BB]

•  This turbulent pressure grows relative to the thermal pressure as Q3/4.

•  It follows that when SNe stir up the ISM, and Q becomes large, then the turbulent pressure will puff up the disk.

•  A disk with a larger scale height has lower density. And the lower density suppresses star formation.


Warm Ionized Medium •  A.ka. Reynolds layer •  Total energy requirement

exceeds the KE from supernova, so OB stars seem like the only viable energy source

•  Requires the equivalent of 1 O4 star (or 15 B0 stars) per kpc2

•  Low ionization parameter consistent with being ~500 pc from the nearest O7 star.

•  About 15% of all ionizing photons leak out of HII regions through holes in the HI distribution

End Lecture 14


the global ism - ucsbweb.physics.ucsb.edu/~phys233/w2016/clm_lecture15_w2016... · 2016. 3. 9. ·...

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